The present invention relates to a rotational phase detecting apparatus for a head drum of a video tape cassette recorder, and more particularly to a rotational phase detecting apparatus for a head drum utilizing a video head which can directly detect the rotational phase of a head drum from a reproducing signal of a video head without using a separate pulse generating head.
In general, in order to produce a head switching signal in a video tape cassette recorder, it has been designed to detect the rotational phase of a head drum.
In a conventional head drum assembly as shown in FIGS. 1A and 1B, there are provided an upper drum 1 provided with first and second video heads 8 and 9, a lower drum 2, a rotor 3 of a drum motor, a rotary shaft 4 fixed to the rotor 3, the upper drum 1 being mounted to the rotary shaft 4, a pair of pulse generating magnets 5 and 6 which are oppositely mounted on the outer peripheral surface of the rotor 3, and a pulse generating head 7 disposed to face the rotational direction of the pulse generating magnets 5 and 6.
By these arrangements, when the pulse generating magnets 5 and 6 pass by the pulse generating head 7 as the rotor 3 of the drum motor rotates, the pulse generating head 7 generates a pulse signal as shown in FIG. 2 and the pulse signal is output as a rotational phase detecting signal for the head drum. That is to say, when the pulse generating magnet 6 reaches the pulse generating head 7, a positive pulse signal is generated from the pulse generating head 7. When the other pulse generating magnet 5 reaches the pulse generating head 7, a negative pulse signal is generated from the pulse generating head 7, and the pulse signal generated from the pulse generating head 7 is applied as a rotational phase detecting signal.
However, in such a conventional apparatus, an angular error may occur when fixing the pulse generating magnets to the rotor of the drum motor, and also an angular error may occur when connecting the rotor to the upper drum by the shaft. Therefore there have been some difficulties in producing the head switching signal from the rotational phase detecting signal for the head drum.